Current collector plates or bipolar plates are an integral part of the fuel cell stack. Current collector plates function in a fuel cell stack as fluid distribution elements and function as bipolar plates to separate MEAs from one another. Each plate assembly is typically constructed of thin gauge metal sheets such as stainless steel coated with a conductive coating. Generally, two adjacent metal sheets are joined together forming a conductive heat exchanger. The joined plate assembly has internal flow channels along its length for coolant flow. Channels on either side of the bipolar plate serve as distribution pathways for fuel and oxidant gas to adjacent current collectors. A hermetic seal is required around the perimeter of the joined plates to prevent mixing of the coolant, fuel, oxidant gas and reactant products. High conductivity must be maintained between the two metal sheets that form the bipolar plate to achieve efficient fuel cell operation.
A conventional method of constructing a bipolar plate has been to first stamp the flow channels into the sheet material and subsequently join the sheets together. Stamping however is unfavorable because it subjects the sheets to high strains and distortion. In addition, joining stamped structures may be complicated. First, after stamping, there is minimal sheet to sheet contact between flow channels, thus, alignment and weld containment are difficult. Secondly, access for joining between the flow channels is obstructed by the flow channel structure.
A variety of joining techniques have been considered to overcome the drawbacks associated with conventional stamping followed by joining. These include brazing, adhesive bonding and fusion welding. These too, however, present unfavorable drawbacks. Brazing of austenitic stainless steel has been successful, but is costly. The cost is partially attributed to the acid cleaning and batch coating necessary after the brazing operation. Both adhesive bonding and fusion welding allow the stainless sheet to be acid cleaned and coated with a continuous coil operation prior to joining which is less costly. Adhesive bonding, however, suffers from durability issues regarding strength and conductivity. Fusion welding of austenitic stainless steel produces a number of potential concerns including distortion, weld cracking, sensitization, stress corrosion cracking, degraded corrosion resistance, damage to the organic coating, sheet alignment, access between flow channels and small contact areas between flow channels for weld containment.